N112-098 TITLE: Gas Turbine Engine Exhaust Jet Shear-layer Pressure Measurement System

TECHNOLOGY AREAS: Air Platform

ACQUISITION PROGRAM: F-35, Joint Strike Fighter

OBJECTIVE: Design, develop, and demonstrate a jet shear-layer hydrodynamic pressure measurement system capable of detecting turbulent large-scale organized structures in full-scale military/commercial engine exhaust plumes.

DESCRIPTION: Laboratory scale supersonic jet noise experimental studies have demonstrated quantitatively that aft far-field noise is controlled by shear-layer large-scale turbulent structures in the form of convected pressure wave packets. Moreover, it has been shown that the wave packet characteristics are well represented by shear-layer instability waves. When the experimentally measured wave packets were projected to the far field, noise levels and spectra were well predicted suggesting that jet noise reduction might be achieved by controlling the spatial-temporal evolution of the shear layer. Confirmation of this shear-layer instability generated jet noise mechanism in full-scale engines is now needed as the next step towards developing advanced noise control strategies. Pursuing engine noise control strategies without such a diagnostic source confirmation would be premature. This is because laboratory nozzle studies have yet to simulate the high turbulence levels, Reynolds numbers, and temperatures typical of full-scale engines.

The system and associated data processing software should be capable of extracting the modal content of the convected hydrodynamic pressure fields during engine performance tests typically conducted at 20 feet (ft) above the ground. Analysis software must be capable of projecting the data to simultaneously acquired acoustic far-field microphones to confirm the noise contribution from organized turbulence structures. The three-dimensional sensor array must extract the flow field modal content for frequencies spanning one decade on either side of the spectral peak at aft jet radiation angles. The resulting calibrated frequency response range would be from approximately 50 Hertz (Hz) to 5 kiloHertz (kHz). The sensors should be located on surface(s) surrounding the spreading hot jet shear layer to ensure that the hydrodynamic pressure field is measured with fidelity. The sensor count should be limited to 160 covering an axial domain of 45-60 ft for a 3-ft diameter engine nozzle. Sensors may be fixed or continuously translated/rotated to discrete locations to minimize sensor count and to allow optimal positioning for reduced order models (e.g. wave packets) resolution requirements. Sound pressure levels of over 160 decibels (dB), hot exhaust flows, radiation heat transfer, and entrained flow over the sensors must be accounted for in selection of the sensors and the mechanical mounting schemes. There may be a need for thermal management on the sensors. Assembly of the system, which must reach engine center line locations on the order of 20 ft above the ground, should be achievable by four technicians in a week. If possible, the system should be capable of folding into a smaller size for outdoor storage on an airfield tarmac when not in use. Structural materials and mechanical/electronic components should be able to withstand rain and temperatures above 32 degrees Fahrenheit.

Near-field holographic array systems have recently been developed by various organizations. However, they are designed to measure the acoustic field at many diameters beyond the boundary of the jet shear-layer hydrodynamic pressure field. The existing holographic arrays are also limited to measuring the noise from stationary aircraft systems with installed engines at approximately 6-7 ft above the ground, rather than isolated engines at 20 ft above the ground at a test stand.

PHASE I: Determine the feasibility of developing a gas turbine engine exhaust jet shear-layer pressure measurement system. Design a concept for the mechanical hardware, drive systems, and instrumentation for the system control and sensors and formulate the software for real-time extraction of the wave packet modal information. Conduct scaled demonstration experiments where necessary to reduce risk.

PHASE II: Develop an integrated and functional jet shear-layer hydrodynamic pressure measurement array and demonstrate it on a military tactical aircraft isolated engine test stand on a piggyback basis. Provide operating procedures, calibration methods, and a real-time software package for detection of the organized structure wave packet modes. Demonstrate the system application via closure between 1) the convected wave packet data and existing instability model predictions and 2) projection of the wave packet data to the acoustic far field for direct comparison with the measured far-field spectrum.

PHASE III: Transition the developed jet shear-layer pressure measurement technology to an original equipment manufacturer (OEM) responsible for the propulsion system of a tactical/civil aircraft, and use it to design/implement "second generation" jet noise reduction hardware for current/future aircraft programs.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Both military and commercial gas turbine engine applications include jet noise source mechanism identification and identification of jet noise reduction concepts.

REFERENCES:
1. Suzuki, T. and Colonius, T., "Instability Waves in a Subsonic Round Jet Detected using a Near-Field Phased Microphone Array," J. Fluid Mech., Vol. 565, pp. 197-226, 2006.

2. Reba, R., Simonich, J.C. and Schlinker, R.H., "Sound Radiated by Large-Scale Wave-Packets in Subsonic and Supersonic Jets," 15th AIAA/CEAS Aeroacoustics Conference, Miami, Florida, May 2009, AIAA 2009-3256.

3. Schlinker, R.H., Reba, R.A, Simonich, J.C., Colonius, T., Gudmundson, K., and Ladeinde, F., "Towards Prediction and Control of Large Scale Turbulent Structure Supersonic Jet Noise," ASME Turbo Expo, June 2009, Orlando, Florida, GT2009-60300.

KEYWORDS: Jet Noise; Exhaust Noise; Turbulent Structure Noise; Shear-Layer Instability; Hydrodynamic Pressure Field; Wave Packets

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